Heatable magnetic stirrers for dissolution processes which have a heatable crucible base of nonmagnetic steel or aluminum are well known. A conventional heatable magnetic stirrer includes a heating plate, having heating elements positioned directly below the lower surface of the plate, and a variable speed motor driving a rotatable permanent magnet. The magnet rotates in a plane parallel to and directly below the heating plate. To use the stirrer, a vessel containing material to be heated is positioned on the heating plate. The vessel is heated by conduction of the heat produced by the heating elements to the heating plate and then to the bottom surface of the vessel. A steel stirring bar is placed in the vessel, and the motor is turned on, rotating the permanent magnet. The rotation of the magnet establishes a rotating magnetic field, which in turn causes the stirring bar to rotate within the vessel, thereby causing its contents to be stirred.
In analytical chemistry, vessels for use at higher temperatures, (i.e. crucibles) made of platinum or glass-carbon (i.e. vitreous carbon), are known. In known heatable magnetic stirrers, heat is transmitted to the crucible at the base of the crucible, from the heating plate surface of the stirrer.
With certain materials that do not readily undergo acid dissolution, such as corundum, titanium ores, ashes, dusts, salts, and the like, high temperatures (above 350.degree. C.) must be achieved rapidly to avoid vaporization of the dissolution liquor which could lead to undesirable alteration of the reaction conditions.
Furthermore, in conventional heatable magnetic stirrers, the temperature gradient from the bottom to the surface of the liquor is relatively high at the temperatures attainable. Because the fluid boils, a fast and high overheating of the liquor is limited. For dissolution of stable samples e.g., corundum or cement, temperatures above the boiling point of the acids are required.
Theoretically it is possible to conduct dissolution of this type by using a magnetic stirrer with a heating plate. Heating plates are usually made of aluminum alloys or vanadium steel. Unfortunately, vanadium steel has an unfavorable coefficient of expansion and has a tendency to deform at temperatures as low as 400.degree. C. The aluminum alloy, e.g., AlSi, loses mechanical stability above 450.degree. C. and becomes soft, which can result in short circuiting of the heating elements.
Moreover, care must be taken in using conventional heatable magnetic stirrers at high tempertures to ensure that heating does not heat up the permanent magnet of the stirrer and thereby reduce its magnetic field. Permanent magnets lose their permanent magnetic properties above 400.degree. C. With conventional heatable magnetic stirrers, design and material limits are exceeded when high temperatures are attained too quickly, even if the high temperatures are maintained for a short period of time. When the crucible is made of platinum, direct contact between the platinum crucible and the heating plate at high temperatures causes diffusion phenomena which result in alloy formation on the platinum surface and in the long run causes localized damage to the heating plate due to melting of the crucible.
Use of a glass-carbon crucible entails the danger of oxidation above temperatures of approximately 400.degree. C. Therefore, rapid heat transfer by a sufficiently highly heated crucible surface leads to oxidation, and thereby destruction, of the crucible.
In addition to the problem of self destruction, high temperature heating in a conventional heatable magnetic stirrer causes contamination of the material being heated. For example, oxidation of the crucible will change the reaction conditons within the material being heated. This presents a particularly large problem when analytical work is conducted.